1. Field of the Invention
This invention relates to acoustic range finding systems of the type in which an electro-acoustic transducer transmits a pulse of acoustic energy towards a surface whose distance is to be measured, and subsequent signals received from the transducer are monitored to determine the temporal location of an echo from that surface.
2. Review of the Art
In practice, problems arise in resolving the wanted true echo from other signals produced by the transducer or its connections. U.S. Pat. No. 4,596,144 of which I am coinventor, describes methods of detecting a true echo in an ultrasonic range finding system which are essentially of a statistical nature, and not only identify an echo resulting from a particular shot but are capable of quantifying the degree of assurance that a selected echo is a true echo. This latter information may be utilized in determining whether additional shots are required to provide reliable data.
All of the echo extraction techniques described in U.S. Pat. No. 4,596,144 have the following steps in common:
1. An echo profile is formed by taking one or more shots, i.e., applying transmit pulses to the transducer, and recording a series of digitized samples of the received signal to form a data base characterizing the echo profile. PA1 2. The first part of the echo profile is blanked in order to cover over the transmit pulse and some transducer ringing. In order to obtain acceptable efficiency, the transducer must have a reasonably high quality factor or Q, and this results in an exponentially decaying oscillation of the transducer which continues after the end of the transmit pulse and initially forms the major portions of the transducer output to a receiver which processes the transducer output. Although the start of the echo profile coincides with the start of the transmit pulse, the useful echo information occurs after the end of blanking. PA1 3. A reference curve is formed. The curve starts at a fixed start point and then follows the profile. PA1 4. The most probably correct echo is selected by comparing the echo profile with the reference curve. PA1 1. An increase or decrease in temperature. PA1 2. A change in the mounting of the transducer; for example, the mounting bolts of the transducer may be tightened. PA1 3. Natural aging of the transducer. PA1 4. Replacement of the transducer.
Certain problems arise in the application of these techniques.
Firstly, it is desirable to set the start point of the reference curve low in order to confidently detect valid close-in echoes. On the other hand it is desirable to set the start point high so that the reference curve will clear the unblanked portion of the transducer ringing following the blanked portion, otherwise the ringing may be deemed to be the correct echo.
In the apparatus described in U.S. Pat. No. 4,596,144, the start point may be set manually by entering a value from the keyboard, or automatically. To set the start point automatically, the operator must first ensure that the material level is well down from the transducer, and then by use of the keyboard instruct the computer to calculate a start point which will cause the reference curve to clear the transducer ringing following the blanking interval. The start point cannot be set with a full bin because the valid closein echo may appear to be transducer ringing and the start point would be set high to clear this echo, with resultant detection of a spurious echo.
A further problem arises because of variations in transducer ringing. The ringing may increase for the following reasons:
The operator must recognize these factors and set the start point high enough to clear the worst case of expected ringing. If the start point is too high then valid close-in echoes will not be detected. If the start point is set too low then the apparatus may initially operate correctly, but a change of season will probably cause an increase in ringing and the start point must then be increased. If a compromise cannot be achieved then the blanking interval must be increased so that less of the ringing is seen. The disadvantage of increasing the blanking is that levels in the top portion of the bin cannot be measured, and the useful height of the bin is thus reduced.
In transmitter design a trade off is made in selecting the transmit pulse width. A narrow pulse width has the effect of shifting the ringing to the left, when viewed graphically, simply because the end of transmission occurs sooner. The position of the echo remains the same and therefore close-in echoes will stand out more above the ringing. A wide transmit pulse has the effect of producing the largest possible return echo, even in the presence of air currents which tend to disperse the sound wave, as often happens with distant targets.
Much effort has been directed to improving transducer performance, but in the present state of the art it is not possible to consistently manufacture a transducer with low and stable ringing while still maintaining other desirable features such as high sound output and rugged construction.
My commonly assigned U.S. Pat. No. 4,831,565, the content of which is incorporated herein by reference, discloses improvements providing the ability to relieve an operator from any involvement in setting the starting point or similar parameter, the ability to have the system continuously and automatically compensate for changes in transducer ringing, the ability to adjust automatically the operation of the system so that close-in echo detection is improved without compromising far echo detection, and the ability to detect defective or absent transducers or transducer connections. The techniques disclosed include the use of shots of short duration combined with special signal processing techniques for detecting close-in echoes which might otherwise be masked by transducer ringing. This provides substantial improvements in close-in performance. A need remains however for improved processing of more distant echoes.
U.S. Pat. No. 4,596,144 discloses several techniques for processing the data stored following a shot or shots so as to identify a desired echo in the presence of spurious echoes, these techniques being described with reference to FIGS. 3, 4 and 5 of the drawings of that patent, and developments of these techniques, particularly in relation to the initial portion of the echo profile, are disclosed in my pending applications already mentioned above. Various problems in echo identification however remain, as follows.
Many bins have false targets in the path of the sound wave. These targets can be in the form of pipes or wires but often consist of seams in the bin wall. In a narrow bin it is usually impossible to move the transducer far enough away to avoid these targets.
In bins containing solids, the material often rests at an angle with several steps in the material. Due to spreading the transducer beam echoes are received from more than one of these steps. Because each step is at a different distance from the transducer a cluster of echoes is received which may be overlapped in time or have narrow gaps between them. In the latter case the echo selection algorithms see these echoes as separate echoes (which they are) even though they should be considered as one echo.
Practical embodiments of my previous inventions have utilized echo processing techniques of the type described with reference to FIG. 4 of U.S. Pat. No. 4,596,144, to provide a time varying threshold or TVT by smoothing the echo profiles, the TVT being utilized as a reference for comparing echoes. I now find that this method of deriving a TVT may not provide optimum performance in compensating for circumstances in which sound attenuation within a bin varies with time, nor in matching the sound attenuation which takes place with increasing bin depth.
With targets which are close to the transducer, the echo produced by the target is partially masked by the transducer ringing. Assuming that the echo rises above the ringing by some amount then it is possible to detect its presence. A problem arises in comparing the strength of this echo with other later echoes in order to select the most likely true echo. In previous arrangements the TVT curve followed the smoothed echo profile so the size of a close-in echo could at best be equal to the amount by which it exceeded the ringing.
It has long been recognized in echo ranging technology that the width of the transmitted pulse can advantageously be adapted to the range being measured. A longer pulse, particularly when using a transmitter transducer with a high quality factor or Q, has increased energy and provides a stronger echo. On the other hand, it limits the minimum range that can be measured, because a short range echo may return before the transmission is completed, or whilst the transducer is still ringing at high amplitude. Various proposals have been made to overcome this problem. Thus U.S. Pat. No. 3,102,261 (Wippert) discloses the technique of varying the transmit pulse length according to the elapsed time to receipt of an echo following a preceding pulse. U.S. Pat. No. 4,000,650 (Snyder) discloses variation of both the pulse length and pulse repetition frequency upon a similar basis: thus the apparatus disclosed normally operates in a short shot mode in which a short pulse is used, but if an echo greater than a certain threshold is not received within a first limited range, a long shot mode is entered, and the largest echo received within a range beyond the first limited range is selected as the true echo. This technique does not overcome the problem that an echo cannot be detected whilst the transducer is still ringing at high amplitude following a pulse, which limits minimum range even with a short transmit pulse. In my U.S. Pat. No. 4,831,565 I disclose techniques for detecting echoes even in the presence of ringing of substantial amplitude, and use a short transmit pulse in combination with these techniques for detecting very short range echoes. A longer pulse is utilized except for detection of these very short range echoes. In the technique disclosed in the Snyder patents, and optionally in my own patent, a `short shot` is taken at the beginning of each ranging sequence, and only if an echo is not detected with a short range is a `long shot` taken. This opens the possibility that a short range spurious echo will prevent a long range measurement from ever being taken, since any short range echo which meets the requirements of the system, will be accepted as a true echo.
In fact, if a `long shot` produces an apparently valid long range echo, then such an echo requires serious consideration as a true echo since such an echo could not normally be produced by a bin or container which is nearly full of material that is not itself a good propagator of acoustic energy. This is recognized in my U.S. Pat. No. 4,831,565, which suggests that in certain circumstances both long and short shots should be used, even when processing of the short shot does reveal an apparently valid echo, and if both shots produce an apparently valid echo, a determination should be made between them.
Some bins contain a fixed undesired target, such as a rough area on the bin wall. This false target can be difficult to distinguish from a true target. A known solution is to place a time window around the false target such that echoes within this window will not be considered in the echo selection process. When the location of the true echo enters this window the last correct reading is held until the location of the true echo leaves the window. There are two problems with this method; firstly, the level is not being measured when it is within the window and secondly, when the level is within the window there may be another strong false echo present outside the window, which will be selected as the true echo.
I have further found that no one echo identification technique which I have investigated uniformly provides the best results under all bin conditions.